Manhattan north of 155th Street was plunged into hot, steamy darkness last night when electric power was lost on Con Edison’s entire Washington Heights network.

The power failure began about 10:20 p.m. and affected 68,000 Con Ed customers – hundreds of thousands of people.

There were smaller power failures last night in other boroughs: about 100 Con Ed customers lost service in scattered parts of the Bronx; some 500 customers were without power in the Williamsburg section of Brooklyn and about 1,300 lines were out in parts of Richmond Hill, Long Island City and Flushing, Queens.

In upper Manhattan, street lights and traffic signals were out, the No. 1 subway was shut down north of 145th Street, and the A train was out north of 168th Street.

Cops from precincts around the city flooded the area, and officials said the area was generally calm.

But there was at least one incident of looting. A crowd of people forced open the gate of a bodega on Sherman Avenue and looted the place.

“I lot of boys broke in and took a lot of money,” said a woman, who identified herself as a friend of the owner. “They took everything.”

Power in part of the neighborhood – the West Side north of 190th Street – had been off much of the day.

The lights in that area came on at around 10:10 p.m. for a few minutes before the total blackout, which a Con Ed spokesman blamed on “excess demand that brought down the Washington Heights-Inwood network.”

Officials weren’t sure when power would be restored.

“There’s no air-conditioning and no fans,” said Henry Marquina, 14, who lives in Inwood. “My whole family is outside. It’s 102 degrees in every apartment.”

“This has been going on since 10 a.m. – no water, no air-conditioning, no lights,” said Carmen Infante, 38. “It’s horrible.”

But many in Washington Heights – which has large numbers of immigrants from the Caribbean – took it in stride.

“In the Dominican Republic, we have more power outages than we have light,” said Robert Gutierrez, who was stuck on a powerless subway train for a half hour.

Columbia Presbyterian Hospital was operating on emergency power. “This is one of the most difficult days the city has ever had with regard to heat and the impact of heat,” said Giuliani, who canceled plans to travel to Nantucket for a Senate campaign fund-raiser.

He said three people died over the past two days – one death confirmed as a result of heat and the other two suspected of being heat-related.

The mayor said a 79-year-old woman died at Mount Sinai Hospital on Monday – and that the temperature in her apartment was greater than 109 degrees.

The heat was also being investigated as a cause in the deaths of a 40-year-old man and a 27-year-old man, both in Brooklyn.

Yesterday’s ferocious 101-degree record scorcher was the second triple-digit reading in as many days.

The worst of the heat wave was expected to be over today.

The twin towers of heat and humidity were expected to crumble to a less misery-producing level.

The high temperature is expected to be in the low 90s – but the big relief will be less humidity, the National Weather Service says.

Tomorrow and Friday, temperatures should drop to the relatively mild mid-80s.

That’ll be a big relief from yesterday, when the thermometer burned its way up to 101 at Central Park at 3:11 p.m., melting the 98-degree record for the date set in 1986. The heat index – the combined punch of heat and humidity – made it feel like a suffocating 110.

Con Edison chairman Gene McGrath said power use yesterday set an all-time high.

“We blew away our past peak,” he said at a press conference with the mayor. “Our previous peak was 11,013 megawatts in 1997. Today was 11,850 megawatts.”

He said Con Ed imposed a 5 percent voltage reduction systemwide at the request of the New York Power Pool, which coordinates utilities statewide.

The mayor said the city also broke another record yesterday – for water use, consuming about 1.8 billion gallons. Normal usage is 1.2 to 1.4 billion.

New Yorkers also broke the record for 911 calls. By 11 p.m., the Fire Department logged 4,696 calls. The previous record of 4,607 was set May 15, 1995.

The heat made for a hellish commute yesterday. Power failures knocked out subway track signals and wreaked havoc on the A and Nos. 1 and 9 lines.

And 40,000 passengers on Metro-North’s New Haven line were backed up because of stalled trains and wires wilted by the heat yesterday morning.